1. Field of the Invention
The present invention relates to a method and device for the continuous mixing and removal of pulverous or fragmentary material from a container, such as a silo or the like.
2. Description of the Prior Art
Mixing different pulverous or fragmentary components before a mechanical, thermal, or chemical treatment is necessary in very many industries, such as the cement and metallurgical industries. In the cement industry in particular there is a great need to mix the powder which is ground in raw-powder mills and which has a chemical composition which varies more or less during the process, into a mixture homogeneous in regard to its chemical composition before the material is fed into the cement furnaces. In the said industry it is also necessary to mix great amounts of clinker or cement prepared at different times in order to lessen the quality fluctuations of the final product.
The known methods for mixing pulverous or fragmentary materials in order to even out the quality can be classified into batch methods and continuous methods.
In continuous methods, an amount of material approximately equal to the amount fed into the silo is continuously removed from the silo. In one known method, pulverous material is continuously kept in a suspended state by means of air blowing, while powder is fed and removed continuously. However, this method requires a considerable air compressor efficiency.
In another known method, in which the mixing efficiency is, however, very low, air is blown through openings at the bottom of the silo so that powder continuously flows out of the silo while powder is fed into the upper part of the silo. Even if the air blowing mechanisms cover most of the silo bottom, the removal of powder takes place through narrow channels directly above the outlet opening, in which case the fed material is carried through these channels directly to the outlet opening without becoming mixed with the bulk of the material in the silo. A certain mixing effect is obtained in these silos if the powder is fed periodically while the removal is taking place continuously. In this case depressions are created on the surface of the material above the outlets, and the upper layers in the silo become mixed with each other when the powder collapses into these depressions. Becuase powder has a relatively low collapsing angle once it has been made to move, the collapsing causes mixing only in a relatively thin layer in the silo.
A better mixing effect is obtained if the homogenization silo has been divided into two layers one on top of the other, in which case the upper one is filled continuously while the lower one is being emptied. When the upper compartment has become filled to a certain degree, the several openings at the bottom of this compartment open and its contents rapidly flow into the lower compartment. In this way a more effective mixing by collapse is effected in the upper compartment while the pulverous material forms a suspended surface layer in the lower compartment and thereby becomes mixed.
The object of the present invention is to eliminate the aforementioned drawbacks and to provide a continuous method and device for removing pulverous or fragmentary material from a container, such as a silo or the like, so that the composition of the outlet flow represents approximately the mean of the compositions of the layers of materials of different compositions in the silo in order to minimize the quality fluctuations of the outlet flow per time unit, and this device is simple and inexpensive both to use and to acquire.